Diffusing alpha-emitter radiation therapy for pancreatic cancer

ABSTRACT

A method for treating a tumor, comprising identifying a tumor as a pancreatic cancer tumor and implanting in the tumor identified as a pancreatic cancer tumor, as least one diffusing alpha-emitter radiation therapy (DaRT) source ( 21 ) with a suitable radon release rate and for a given duration, such that the source ( 21 ) provides during the given duration a cumulated activity of released radon between 5.6 Mega becquerel (MBq) hour and 11.6 MBq hour, per centimeter length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT application PCT/IB2022/055324,filed Jun. 8, 2022, which is a continuation-in-part of U.S. patentapplication Ser. No. 17/343,779, filed Jun. 10, 2021. The disclosures ofthese related applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to radiotherapy and particularlyto apparatus and methods for providing tumor-specific radiation dosagesin radiotherapy treatment.

BACKGROUND OF THE INVENTION

Ionizing radiation is commonly used in the treatment of certain types oftumors, including malignant cancerous tumors, to destroy their cells.Ionizing radiation, however, can also damage healthy cells of a patient,and therefore care is taken to minimize the radiation dose delivered tohealthy tissue outside of the tumor, while maximizing the dose to thetumor.

Ionizing radiation destroys cells by creating damage to their DNA. Thebiological effectiveness of different types of radiation in killingcells is determined by the type and severity of the DNA lesions theycreate. Alpha particles are a powerful means for radiotherapy since theyinduce clustered double-strand breaks on the DNA, which cells cannotrepair. Unlike conventional types of radiation, the destructive effectof alpha particles is also largely unaffected by low cellular oxygenlevels, making them equally effective against hypoxic cells, whosepresence in tumors is a leading cause of failure in conventionalradiotherapy based on photons or electrons. In addition, the short rangeof alpha particles in tissue (less than 100 micrometers) ensures that ifthe atoms which emit them are confined to the tumor volume, surroundinghealthy tissue will be spared. On the other hand, the short range ofalpha radiation has so far limited their use in cancer therapy, as therewas no practical way to deploy alpha emitting atoms in sufficientconcentrations throughout the entire tumor volume

Diffusing alpha-emitters radiation therapy (DaRT), described for examplein U.S. Pat. No. 8,834,837 to Kelson, extends the therapeutic range ofalpha radiation, by using radium-223 or radium-224 atoms, which generatechains of several radioactive decays with a governing half-life of 3.6days for radium-224 and 11.4 days for radium-223. In DaRT, the radiumatoms are attached to a source (also referred to as a “seed”) implantedin the tumor with sufficient strength such that they do not leave thesource in a manner that they go to waste (by being cleared away from thetumor through the blood), but a substantial percentage of their daughterradionuclides (radon-220 in the case of radium-224 and radon-219 in thecase of radium-223) leave the source into the tumor, upon radium decay.These radionuclides, and their own radioactive daughter atoms, spreadaround the source by diffusion up to a radial distance of a fewmillimeters before they decay by alpha emission. Thus, the range ofdestruction in the tumor is increased relative to radionuclides whichremain with their daughters on the source.

In order for the treatment of a tumor to be effective, DaRT seedsemployed in the treatment should release a sufficient number of radonatoms to destroy the tumor with a high probability. If an insufficientamount of radiation is employed, too many cancerous cells will remain inthe tumor, and these cells may reproduce to reform the malignant tumor.On the other hand, the seeds should not release too many radon atoms, assome of their daughters are cleared from the tumor through the blood andcould therefore damage distant healthy tissue, including organs such asbone marrow, kidneys and/or ovaries of a patient.

The amount of radium atoms on the DaRT source is quantified in terms ofthe activity, i.e., the rate of radium decays. The DaRT source activityis measured in units of micro-Curie (μCi) or kilo-Becquerel (kBq), where1 μCi=37 kBq=37,000 decays per second. When using DaRT, the radiationdose delivered to the tumor cells depends not only on the radiumactivity of the source, but also on the probability that the daughterradon atoms will leave the source into the tumor, upon radium's alphadecay. This probability is referred to herein as the “desorptionprobability”. Thus, instead of referring to the activity of the source,one can use the “radon release rate”, which is defined herein as theproduct of activity on the source and the desorption probability ofradon from the source, as a measure of the DaRT related activity of asource. Like the activity, the radon release rate is given in μCi orkBq. The activity and radon release rate values given herein are, unlessstated otherwise, of the source at the time of implantation of thesource in the tumor

The above mentioned U.S. Pat. No. 8,834,837 to Kelson suggests using anactivity “from about 10 nanoCurie to about 10 microCurie, morepreferably from about 10 nanoCurie to about 1 microCurie.”

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to providing accuratelytailored amounts of radiation to a tumor in a radiotherapy treatment.The embodiments include radiotherapy sources designed to providesuitable amounts of radiation, and kits including a suitable number ofsources for tumors of specific sizes. Further embodiments relate tomethods of preparing kits of radiotherapy sources for a specific tumorand methods of treatment of a tumor.

There is therefore provided in accordance with an embodiment of thepresent invention, a method for treating a tumor, comprising identifyinga tumor as a pancreatic cancer tumor and implanting in the tumoridentified as a pancreatic cancer tumor, at least one diffusingalpha-emitter radiation therapy (DaRT) source with a suitable radonrelease rate and for a given duration, such that the source providesduring the given duration a cumulated activity of released radon between5.6 Mega becquerel (MN) hour and 11.6 MN hour, per centimeter length.

Optionally, implanting the at least one radiotherapy source comprisesimplanting an array of sources, each source separated from itsneighboring sources in the array by not more than 4 millimeters.Optionally, implanting the at least one radiotherapy source comprisesimplanting an array of sources in a hexagonal arrangement, each sourceseparated from its neighboring sources in the array by not more than 4millimeters. Optionally, the at least one radiotherapy source has aradon release rate of between 1.2 and 2.5 microcurie per centimeterlength. Optionally, the at least one radiotherapy source has a radonrelease rate of between 1.4 and 1.9 microcurie per centimeter length.Optionally, the method comprises selecting the given duration beforeimplanting the at least one DaRT source in the tumor, and removing thesources from the tumor after the given duration from the implanting ofthe sources passed.

There is further provided in accordance with an embodiment of thepresent invention, a method of preparing a radiotherapy treatment,comprising identifying a tumor as a pancreatic cancer tumor, receivingan image of the tumor; and providing a layout of diffusing alpha-emitterradiation therapy (DaRT) sources for the pancreatic cancer tumor,wherein the sources have a radon release rate of between 1.2 and 2.5microcurie per centimeter length. Optionally, providing the layoutcomprises providing a layout in which a spacing between sources in thetumor is 4 millimeters or less. Optionally, the sources have a radonrelease rate of between 1.4 and 1.9 microcurie per centimeter length.

There is further provided in accordance with an embodiment of thepresent invention, an apparatus for preparing a radiotherapy treatment,comprising an input interface for receiving information on a tumor, aprocessor configured to determine that the tumor is a pancreatic cancertumor and to generate a layout of diffusing alpha-emitter radiationtherapy (DaRT) sources for the tumor, wherein the sources in the layouthave a radon release rate of between 1.2 and 2.5 microcurie percentimeter length and the sources in the layout are arranged in aregular pattern having a distance between adjacent sources of not morethan 4 millimeters and an output interface for displaying the layout toa human operator.

There is further provided in accordance with an embodiment of thepresent invention, a method of preparing a radiotherapy treatment,comprising receiving a request for diffusing alpha-emitter radiationtherapy (DaRT) sources for a pancreatic cancer tumor, determining anumber of radiotherapy sources required for the pancreatic cancer tumor;and providing a sterile kit including the determined number ofradiotherapy sources, wherein the sources have a radon release rate ofbetween 1.2 and 2.5 microcurie per centimeter length.

Optionally, determining the number of required radiotherapy sourcescomprises determining a number of sources required such that the area ofthe tumor is covered by sources with a spacing between the sources whichis not greater than 4 millimeters. Optionally, the sources have a radonrelease rate of between 1.4 and 1.9 microcurie per centimeter length.

There is further provided in accordance with an embodiment of thepresent invention, a diffusing alpha-emitter radiation therapy (DaRT)source for implantation in a pancreatic cancer tumor, wherein the DaRTsource has a radon release rate of between 1.2 and 2.5 microcurie percentimeter length. Optionally, the radon release rate is between 1.4 and1.9 microcurie per centimeter length.

There is further provided in accordance with an embodiment of thepresent invention, a kit of diffusing alpha-emitter radiation therapy(DaRT) source for implantation in a pancreatic cancer tumor, comprisinga sterile package; and a plurality of DaRT sources placed in the sterilepackage, the sources having a radon release rate of between 1.2 and 2.5microcurie per centimeter length. Optionally, the radon release rate ofthe sources is between 1.4 and 1.9 microcurie per centimeter length.

There is further provided in accordance with an embodiment of thepresent invention, a method for treating a tumor, comprising identifyinga tumor as a pancreatic cancer tumor; and implanting in the tumoridentified as a pancreatic cancer tumor, an array of diffusingalpha-emitter radiation therapy (DaRT) sources, in a regular arrangementhaving a spacing between each two adjacent sources of between 3 and 4millimeters. Optionally, implanting the array of sources comprisesimplanting in a hexagonal arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for planning aradiotherapy treatment, in accordance with an embodiment of the presentinvention;

FIG. 2 is a flowchart of acts performed in preparing a radiotherapytreatment of a tumor, in accordance with an embodiment of the invention;

FIG. 3 is a schematic illustration of a regular arrangement of sourcesin a hexagonal arrangement, in accordance with an embodiment of theinvention;

FIG. 4 is a schematic illustration of a kit of DaRT sources, inaccordance embodiments of the present invention;

FIG. 5 is a schematic illustration of a radiotherapy source, inaccordance with an embodiment of the present invention;

FIGS. 6A-6D are graphs which illustrate the wide range of radon releaserate values, required to ensure a nominal alpha-particle dose of atleast 10 Gray (Gy), for different seed spacings, lead-212 leakageprobabilities and radon-220 and lead-212 diffusion lengths;

FIG. 6E is a contour graph showing the value of the required radonrelease rate for a 4 mm spacing, 50% lead-212 leakage, and a radiationdose of 10 Gy, for various possible radon-220 and lead-212 diffusionlengths over a range of interest, in accordance with embodiments of theinvention;

FIG. 6F is a contour graph showing the minimal radiation dose expectedto reach the cells of a tumor in which seeds of 3 microcurie per cmlength are implanted at a 4 mm spacing, assuming 50% lead-212 leakage,for various possible radon-220 and lead-212 diffusion lengths, inaccordance with embodiments of the invention;

FIG. 7 is a graph which shows a safety factor for various spacings and arange of radon release rates for pancreatic cancer, in accordance withembodiments of the present invention;

FIG. 8A shows a spatial distribution of the photo-stimulatedluminescence (PSL) signal in a histological section of a 4T1 tumor withdenoted region of sampled data in white (0-4 mm) and the fit region withmagenta dashed line (0.5-3 mm);

FIG. 8B is a graph of the radial activity distribution for the sampleddata of FIG. 8A, fitted by a theoretical model to extract the effectivediffusion length;

FIG. 8C shows a PSL spatial distribution in another histological sectionfrom the same tumor where the seed position is determined automaticallyby calculating the intensity center-of-gravity;

FIG. 8D is a graph of the radial activity distribution for the sampleddata of FIG. 8C, fitted by a theoretical model to extract the effectivediffusion length;

FIGS. 9-14 show the obtained effective diffusion lengths for differenttumor types as a function of the tumor mass;

FIG. 15 shows Four histological sections of a DaRT treated tumor,acquired using a digital autoradiography system, for measurement of theradon-220 diffusion length;

FIG. 16A shows a single histological section used for measurement of theradon-220 diffusion length; and

FIG. 16B shows calculated average counts as a function of distance froma seed location, with a fitted model used for measurement of theradon-220 diffusion length.

DETAILED DESCRIPTION OF EMBODIMENTS

An aspect of some embodiments of the invention relates to setting radonrelease rates of DaRT sources used in treating different types of tumorsaccording to characteristics of the tumors. Applicant has created amodel which estimates the dose reaching the cells of a tumor, as afunction of the diffusion length of lead-212 in the tumor, the diffusionlength of radon-220 in the tumor and the leakage probability oflead-212. The diffusion length represents the typical distance from thepoint an atom was created in the decay of its parent radionuclide to thepoint where it decays. It determines the spatial distribution of thediffusing atoms around the seed; when the radial distance from the seedincreases by one diffusion length, the alpha particle dose drops byapproximately a factor of 3. For seeds with radon release ratesconsidered here, the diameter of the region receiving an alpha particledose of 10 Gy around the seed is roughly 10 times larger than thediffusion length. Methods of measuring an effective diffusion length andhence estimating the diffusion length of radon-220 and a range of valuesof the lead-212 diffusion length are described in the appendices. Theleakage probability of lead-212 represents the chances that a lead-212atom released from the source will leave a tumor through the bloodsystem before it decays.

The diffusion lengths of radon-220 and the leakage probability oflead-212 have different values in different types of cancer tumors.Generally, the shorter the diffusion length of radon-220, the moreactivity is required to achieve similar results. Applicant has estimatedthe diffusion length of radon-220 in various types of tumors andaccordingly has determined radon release rates of sources to be used intreating these tumor types.

FIG. 1 is a schematic illustration of a system 100 for planning aradiotherapy treatment, in accordance with an embodiment of the presentinvention. The treatment generally includes implantation of a pluralityof sources in a tumor which is to be destroyed. System 100 comprises animaging camera 102 which acquires images of tumors requiringradiotherapy. In addition, system 100 includes an input interface 104,such as a keyboard and/or mouse, for receiving input from a humanoperator, such as a physician. Alternatively or additionally, system 100comprises a communication interface 106 for receiving instructionsand/or data from a remote computer or human operator. System 100 furthercomprises a processor 108 configured to generate a layout plan ofradiotherapy sources in the tumor and accordingly to provide through anoutput interface 110, details of respective kits of radiotherapy sourcesfor treatment of the tumors. Output interface 110 may be connected to adisplay and/or to a communication network. Processor 108 optionallycomprises a general purpose hardware processor configured to runsoftware to execute its tasks described hereinbelow. Alternatively oradditionally, processor 108 comprises a dedicated processor, such as asignal processing processor, a digital signal processor (DSP) or avector processor, configured with suitable software for performing itstasks described herein. In other embodiments, processor 108 comprises adedicated hardware processor configured in hardware, such as an FPGA orASIC to perform its tasks.

In some embodiments, processor 108 is further configured to estimate theradiation dose expected to reach each of the points in the tumor, forexample as described in PCT application PCT/IB2021/050034, filed Jan. 5,2021, and titled “Treatment Planning for Alpha Particle Radiotherapy”,the disclosure of which is incorporated herein by reference.

FIG. 2 is a flowchart of acts performed in preparing a radiotherapytreatment of a tumor, in accordance with an embodiment of the invention.The method of FIG. 2 generally begins with system 100 receiving (202)input on the tumor such as an image of the tumor and/or a type of thetumor. A spacing between the sources to be inserted to the tumor isselected (204) for the tumor and accordingly a number of sources to beincluded in a treatment kit for the tumor is determined (206). Inaddition, a duration of the treatment is selected (208). The radonrelease rate of the sources is also selected (210). In some embodiments,instructions on a layout of the sources in the tumor are also prepared(212). Thereafter, a kit including the number of sources of the selectedparameters is prepared (214) and packaged in a suitable sterile package.In some embodiments, the method further includes the treatmentprocedure. In those embodiments, the method includes implanting (216)the sources from the kit into the tumor, for example in accordance withthe prepared (212) layout. In some embodiments, the method includesremoving (218) the sources after the selected (208) duration. In otherembodiments, the sources are not removed and remain in the patient.

In some embodiments, the type of the tumor is determined based onclinical and/or histopathological observations, such as an analysis of aportion of the tumor taken in a biopsy and/or an amount and/or densityof blood vessels in the tumor as determined from an image of the tumor.The type of the tumor is selected, for example, from a list includingsquamous cell carcinoma, basal cell carcinoma, glioblastoma, sarcoma,pancreatic cancer, lung cancer, prostate cancer, breast cancer andcolorectal cancer.

In some embodiments, the sources are arranged in the layout in a regulargeometrical pattern which achieves a relatively low distance betweeneach point in the tumor and at least one of the sources.

FIG. 3 is a schematic illustration of a regular arrangement of sourcesin a hexagonal arrangement 160, in accordance with an embodiment of theinvention. In hexagonal arrangement 160, a surface through which sourcesare entered into a tumor to be treated is divided into hexagons 164 andthe center 162 of each hexagon is designated for insertion of a source.The centers 162 for insertion of the sources are located at the verticesof equilateral triangles distance 166 between each two sources isreferred to herein as the spacing of the layout. The hexagons 164 areformed by bisectors to the lines connecting the centers 162 to their sixnearest neighboring centers 162. The smallest dose of radiation from thesources is at the center of gravity of the triangles, which are at thehexagon vertices. Optionally, the spacing between the sources is smallerthan 5 millimeters, not greater than 4.5 millimeters, not greater than 4millimeters, not greater than 3.5 millimeters, or even not greater than3 millimeters. The spacing between the sources is highly significant indetermining a treatment plan for a specific cancer type, as discussedhereinbelow.

The spacing between the sources is optionally selected (204) as acompromise between the desire to ensure destruction of the tumor withoutusing activity levels which could be close to safety limits, whichpushes for a small spacing, and simplicity of the implantationprocedure, which pushes for a larger spacing. Generally, the largestspacing which is still believed to destroy the tumor with seeds havingan activity level which is not too high, is selected. The spacing isselected (204) responsively to the type of the tumor, because radon-220and lead-212 have different diffusion lengths in different tumor types,and therefore DaRT sources have different effective ranges in differenttumor types. In addition, different tumor types have different requiredradiation doses. In some embodiments, the spacing is selected (204)according to a type of treatment of the tumor. One type of treatment isdirected to complete destruction of the cells of the tumor. Another typeof treatment is directed to reduction of the mass of the tumor to a sizethat is not visible by a naked eye, or to a size that will make thetumor resectable. Complete destruction generally requires a higheractivity level of the sources and/or a smaller spacing between thesources.

Alternatively or additionally, in selecting (204) the spacing, theaccessibility of the location of the tumor within the patient's body istaken into consideration. For example, for tumors in internal organswhich need to be accessed by a catheter or an endoscope, a largerspacing is preferred than for similar tumors which are easily accessed.In some embodiments, the spacing between the sources is selected whiletaking into consideration the time and complexity of implantation of thesources. The smaller the spacing, the more sources are required andaccordingly the time of implantation of the sources increases.Therefore, in accordance with some embodiments of the invention, thelargest spacing that would still allow for destruction of the tumor, isused.

FIG. 4 is a schematic illustration of a kit 700 of DaRT sources 21 inaccordance with embodiments of the present invention. Kit 700 comprisesa sterile package 702 including a plurality of alpha-emitterradiotherapy sources 21, for insertion into a tumor.

Optionally, the sources 21 are provided within a vial or other casing706 which prevents radiation from exiting the casing. In someembodiments, the casing is filled with a viscous liquid, such asglycerine, which prevents radon atoms from escaping the casing 706, suchas described in PCT application PCT/IB2019/051834, titled “RadiotherapySeeds and Applicators”, the disclosure of which is incorporated hereinby reference. In some embodiments, kit 700 further includes a seedapplicator 708, which is used to insert sources 21 into the patient, asdescribed in PCT application PCT/IB2019/051834. Optionally, applicator708 is provided preloaded with one or more sources 21 therein. Inaccordance with this option, separate sources 21 in casings 706 aresupplied for cases in which more than the number of preloaded sources isrequired. Alternatively, sources 21 in casings 706 are not provided inkit 700 and only sources within applicator 708 are included in the kit700.

The number of sources to be included in a treatment kit 700 for thetumor is determined (206) according to the selected spacing and sourcelayout, in order to cover the entire tumor. In some embodiments, anextra 10-20% sources are provided in the treatment kit.

The duration of the treatment (e.g., the time that the seeds remain inthe tumor) is optionally selected by the operator, according to adesired treatment (e.g., complete destruction, mass reduction). In someembodiments, the duration of the treatment is selected (208) in advancebased on parameters of the tumor such as its location in the patient'sbody and the availability of the patient for removal of the sources.Alternatively, the duration of the treatment is selected (208) duringthe treatment, based on the progress of the treatment.

The activity of the sources and their desorption probability areoptionally selected (210) responsive to the selected spacing, thetreatment duration and the tumor type. In some embodiments, the activityof the sources and their desorption probability are further selectedresponsive to a type of treatment of the tumor. If, for example, anoperator indicates that a complete destruction of the cells of the tumoris to be aimed for, a higher activity and/or desorption probability isused than for an indication that a removal of the tumor from naked eyesurveillance, or a reduction of the tumor size to make it resectable, isrequired. Optionally, the activity and source probability are selectedwith an aim to achieve at least a specific radiation dose at each pointthroughout the tumor (or at least at above a threshold percentage ofpoints throughout the tumor), according to the type of the tumor, asdiscussed in more detail below.

It is noted that while the risk of an overdose of radiation for a singlesmall tumor is low, when treating large tumors and/or multiple tumors,the treatment may include implantation of several hundred sources. Insuch cases, it is important to accurately adjust the activity of thesources to prevent administering an overdose of radiation to thepatient. It is generally considered undesirable to implant in a patientan activity level of more than several (e.g., 2-5) millicurie. However,to be on the safe side, a limit of about 1 millicurie is currently used.For a large tumor requiring 170 centimeters or more of seeds, this setsa limit of about 6 microcurie on the activity of a single centimeterlength of a seed. In terms of radon release rate, given a desorptionrate of 38-45%, this sets a limit of about 2.5 microcurie. This limit isnot the same for all tumor types. Some tumor types, such as glioblastomamultiforme (GBM), prostate, breast and squamous cell carcinoma, aregenerally expected to be treated by radiation when they are small.Therefore, the number of seeds used and their total length are expectedto be smaller than 170, so that higher radon release rates may be used.Other cancer types, such as pancreas, are expected to require radiationtreatment for large tumors. Further cancer types, such as melanoma andcolorectal, are expected to require radiation treatment for severaldifferent tumors. These cancer types may require seeds at a total lengthof 170 cm or even more.

It is noted that the acts of FIG. 2 are not necessarily performed in theorder in which they are presented. For example, in cases in which theactivity of the sources is not selected (210) responsive to thetreatment duration, the activity of the sources may be selected (210)before, or in parallel to, selecting (208) the treatment duration. Asanother example, the preparation of the layout and the preparation ofthe kit may be performed concurrently or in any desired order.

FIG. 5 is a schematic illustration of a radiotherapy source 21, inaccordance with an embodiment of the present invention. Radiotherapysource 21 comprises a support 22, which is configured for insertion intoa body of a subject. Radiotherapy source 21 further comprisesradionuclide atoms 26 of radium-224 on an outer surface 24 of support22, as described, for example, in U.S. Pat. No. 8,894,969, which isincorporated herein by reference. It is noted that for ease ofillustration, atoms 26 as well as the other components of radiotherapysource 21, are drawn disproportionately large. Atoms 26 are generallycoupled to support 22 in a manner such that radionuclide atoms 26 do notleave the support, but upon radioactive decay, their daughterradionuclides, shown symbolically as 28, may leave support 22 due torecoil resulting from the decay. The percentage of daughterradionuclides 28 that leave the support due to decay is referred to asthe desorption probability. The coupling of atoms 26 to support 22 isachieved, in some embodiments, by heat treatment. Alternatively oradditionally, a coating 33 covers support 22 and atoms 26, in a mannerwhich prevents release of the radionuclide atoms 26, and/or regulates arate of release of daughter radionuclides 28, upon radioactive decay.Daughter radionuclides may pass through coating 33 and out ofradiotherapy source 21 due to recoil or the recoil may bring them intocoating 33, from which they leave by diffusion. In some embodiments, asshown in FIG. 5 , in addition to coating 33, an inner coating 30 of athickness T1 is placed on support 22 and the radionuclide atoms 26 areattached to inner coating 30. It is noted, however, that not allembodiments include inner coating 30 and instead the radionuclide atoms26 are attached directly to support 22.

Support 22 comprises, in some embodiments, a seed for complete implantwithin a tumor of a patient, and may have any suitable shape, such as arod or plate. Alternatively to being fully implanted, support 22 is onlypartially implanted within a patient and is part of a needle, a wire, atip of an endoscope, a tip of a laparoscope, or any other suitableprobe.

In some embodiments, support 22 is cylindrical and has a length of atleast 1 millimeter, at least 2 millimeters, or even at least 5millimeters. Optionally, the seeds have a length of between 5-60 mm(millimeters). Support 22 optionally has a diameter of 0.7-1 mm,although in some cases, sources of larger or smaller diameters are used.Particularly, for treatment layouts of small spacings, support 22optionally has a diameter of less than 0.7 mm, less than 0.5 mm, lessthan 0.4 mm or even not more than 0.3 mm.

The activity on support 22 is measured herein in units of microcurie percentimeter length of the source. As the radiation dose reaching most ofthe tumor is dominated by radionuclides that leave the source, a measureof “radon release rate” is defined herein as the product of activity onthe source and desorption probability. For example, a source with 2microcurie activity per centimeter length and a 40% desorptionprobability has a radon release rate of 0.8 microcurie per centimeterlength.

The desorption probability depends on the depth of radionuclide atoms 26within the surface of support 22 and/or on the type and thickness ofcoating 33. The implanting of the radionuclide atoms 26 in the surfaceof support 22 is generally achieved by heat treatment of theradiotherapy device 21, and the depth of atoms 26 is controllable byadjusting the temperature and/or duration of the heat treatment. In someembodiments, the desorption probability is between about 38-45%.Alternatively, higher desorption probabilities are achieved, for exampleusing any of the methods described in PCT publication WO 2018/207105,titled: “Polymer Coatings for Brachytherapy Devices”, the disclosure ofwhich is incorporated herein by reference. In other embodiments, lowerdesorption probabilities are used, such as described in US provisionalpatent application 63/126,070, titled: “Diffusing Alpha-emittersRadiation Therapy with Enhanced Beta Treatment”, the disclosure of whichis incorporated herein by reference.

It is noted that not all the alpha radiation that reaches the tumor isdue to daughter radionuclides 28 of radon-220 that leave the support 22upon decay. Some of the daughter radionuclides 28 of radon-220 generatedfrom decay of radionuclide atoms 26, remain on support 22. When thedaughter radionuclides 28 decay, their daughter radionuclides, e.g.,plotonium-216, may leave the support 22 due to recoil, or lead-212generated upon decay of plotonium-216 may leave support 22 due torecoil.

Generally, radionuclide atoms 26 are coupled to support 22 in a mannerwhich prevents the radionuclide atoms 26 themselves from leaving support22. In other embodiments, radionuclide atoms 26 are coupled to support22 in a manner which allows radionuclide atoms 26 to leave the supportwithout decay, e.g., by diffusion, for example using any of the methodsdescribed in PCT publication WO 2019/193464, titled: “Controlled Releaseof Radionuclides”, which is incorporated herein by reference. Thediffusion is optionally achieved by using a bio-absorbable coating whichinitially prevents premature escape of radionuclide atoms 26 but afterimplantation in a tumor disintegrates and allows the diffusion.

The total amount of radiation released by a source in a tumor, referredto herein as “cumulated activity of released radon”, depends on theradon release rate of the source and the time for which the sourceremains in the tumor. If the source is left in the tumor for a longperiod, for example more than a month for a radium-224 source, thecumulated activity of released radon reaches the product of radonrelease rate of the source multiplied by the mean life time ofradium-224, which is 3.63 days or 87.12 hours, divided by ln2, which isabout 0.693. For example, a radium-224 source having a radon releaserate of 1 microCurie (μCi)=37,000 becquerel (Bq), has a cumulatedactivity of released radon of about 4.651 Mega becquerel (MBq) hour. Itis noted that the same amount of cumulated activity of released radonmay be achieved by implanting a source with a higher radon release ratefor a shorter period. For such a shorter period, the cumulated activityis given by:

${{cumulated}{activity}} = {{S(0)}*\tau*\left( {1 - e^{- \frac{t}{t}}} \right)}$

where S(0) is the radon release rate of the source when it is insertedinto the tumor, r is the mean radium-224 lifetime and t is the treatmentduration in hours. For example, a two-week treatment provides acumulated activity of:

${{cumulated}{{activity}\left( {14{days}} \right)}} = {{0.037{MBq}*125.7h*\left( {1 - e^{- 14*\frac{24}{125.7}}} \right)} = {4.33{MBqh}}}$

The required amount of activity on the sources in order to achieve tumordestruction varies dramatically for different types of tumors and sourcespacings. It is therefore important to identify for each type of tumor,the required activity for that specific tumor type. Methods forcalculating the radiation dose reaching each point in a tumor, accordingto the activity of the implanted sources, are described in U.S. patentapplication Ser. No. 17/141,251, filed Jan. 5, 2021 and titled,“Treatment Planning for Alpha Particle Radiotherapy”, the disclosure ofwhich is incorporated herein by reference. Using those calculationmethods, the required radon release rate can be calculated as a functionof the diffusion length of lead-212 in the tumor, the diffusion lengthof radon-220 in the tumor, the spacing between the sources implanted inthe tumor, the leakage probability of lead-212 from the tumor and theradiation dose required to reach each location in the tumor.

FIGS. 6A-6D are graphs which illustrate the wide range of radon releaserate values, required to ensure a nominal alpha-particle dose of atleast 10 Gray (Gy), for different values of the above parameters. The 10Gy level is chosen as a reference, as the nominal alpha particle doserequired depends on the tumor type and can be as high as 20-30 Gy. Toget the required seed activity for a target dose other than 10 Gy, theseed activity for 10 Gy should be multiplied by the ratio between thetarget dose and 10 Gy. FIG. 6A shows the required radon release rate asa function of the lead-212 diffusion length, for three different valuesof the radon-220 diffusion length, when the lead leakage probability is80% and the spacing is 3.5 mm FIG. 6B is a similar graph, for a leadleakage probability of 40%. FIG. 6C shows the same graph for a spacingof 4 mm and lead leakage probability of 80%, while FIG. 6D shows therequired radon release rate for 4 mm spacing and 40% lead leakageprobability. The reader will appreciate that the range of possible radonrelease rate values is very large and the following discussion providesguidance as to narrow ranges to be used for specific tumor types.

FIG. 6E is a contour graph showing the value of the required radonrelease rate for a 4 mm spacing, 50% lead-212 leakage, and a radiationdose of 10 Gy, for various possible radon-220 and lead-212 diffusionlengths over a range of interest, in accordance with embodiments of theinvention.

FIG. 6F is a contour graph showing the minimal radiation dose expectedto reach the cells of a tumor in which seeds of 3 microcurie per cmlength are implanted at a 4 mm spacing, assuming 50% lead-212 leakage,for various possible radon-220 and lead-212 diffusion lengths, inaccordance with embodiments of the invention.

As can be seen in FIG. 6E, the required radon release rate variessubstantially for different diffusion lengths. As different tumor typeshave different diffusion lengths, the required radon release rate isdifferent for different tumor types.

In order to estimate the diffusion length of lead-212 and the diffusionlength of radon-220 in different types of tumors, applicant performedtwo classes of experiments, on various types of tumors and on tumors ofvarious sizes. In a first experiment class, applicant implanted sourcesinside tumors generated in mice and after a few days dissected the tumorand measured the actual activity that reached the various points in thetumor. These measurements are fit into the above equations andaccordingly an effective long-term diffusion length in the tumor isestimated. This effective diffusion length is the larger of thediffusion lengths of radon-220 and lead-212.

The tumor was removed from the mouse and frozen so that the tumor can besliced a short time after the removal of the tumor from the mouse.Thereafter, the tumor was cut into slices of a thickness of about 10microns. Fixation by formalin was done immediately after sectioning, andfor a short duration (minutes), directly on the histological slices,placed on glass slides. After fixation, the slides were laid on a Fujiphosphor imaging plate in closed box for one hour. The slides wereseparated from the plate by a thin Mylar foil to avoid contaminating theplate by radioactivity. The plate was subsequently scanned by a phosphorimaging autoradiography system (Fuji FLA-9000) to record the spatialdistribution of lead-212 inside the histological slices.

Further details of the measurement of the effective long-term diffusionlength are discussed hereinbelow in appendix A.

The second class of experiments is similar to the first class, butrather than waiting several days, the tumor was removed about half anhour after source insertion. The distribution of radioactivity aftersuch a short duration is believed to be predominantly due to diffusionof radon-220, as the spatial distribution of radon-220 stabilizes veryfast, while the contribution arising from lead-212 increases from zeroto a maximal value about 1.5-2 days after source insertion, and issufficiently low 30 minutes after source insertion. Details of themeasurement of the diffusion length of radon-220 are discussedhereinbelow in appendix B.

Early measurements of the diffusion length of radon-220 found values ofbetween 0.23 and 0.31 mm. The number of measurements, however, wasrelatively small. Recent results of the above described measurementsshowed, surprisingly, no significant difference between the long-termand short-term experiments. Applicant therefore postulates that thediffusion length of lead-212 is smaller than the diffusion length ofradon-220. Applicant is therefore assuming that the lead-212 is about0.2 millimeters. This assumption is being used because, as can be seenin FIG. 6E, the dependence on the lead-212 diffusion length is weak inthe range of values of the diffusion length of radon-220. The measuredradon-220 diffusion lengths are summarized for a plurality of cancertypes in the following table 1.

As is known in the art, different tumor types require different doses ofradiation for destruction of their cells. Table 1 includes the requiredbiological effective dose (BED) of various types of cancer tumors inGray equivalent (GyE). These dose values are for photon-based radiation(x-, or gamma-rays). Alpha radiation is considered more lethal to cells,and therefore the dose of alpha radiation in Gray is multiplied by acorrection factor known as relative biological effect (RBE), currentlyestimated as 5, to convert it to BED in Gray equivalent (GyE). The BEDin DaRT is the sum of the alpha dose multiplied by the RBE and the betadose arising from radium-224 and its daughters.

The lead-212 leakage probability is relatively low in the center of thetumor, but reaches about 80% on the periphery of the tumor. In order toensure cell destruction throughout the tumor, applicant has used the 80%leakage probability value in selecting the radon release rate of thesources.

In order to estimate the desired spacing and radon release rate of theseeds for a specific tumor type, applicant estimates the required dosefor the tumor type, the beta radiation dose provided by a span ofactivity levels, and a remaining required dose that needs to be providedby the alpha radiation. The alpha radiation dose is estimated for a spanof spacings and radon release rates and a safety factor which is theratio between the estimated provided dose and the required dose iscalculated for the span of spacings and radon release rates. The safetyfactor is required to overcome inaccuracies which may occur in theplacement of the sources, such that some sources may be separated by anextent larger than the prescribed spacing. In addition, the tumor may benon-homogenous with some local variations in the diffusion lengths.

Applicant has selected the safety factor range of between 1.5-4 asdefining the desired spacing and radon release range for treatment. Thissafety factor is believed to provide sufficient safety that the tumorwill be destroyed by the provided radiation, while not being too high torisk the patient from systemic radiation, arising from the leakage oflead-212 from the tumor through the blood and subsequent uptake invarious organs.

For a given tumor type, the same safety factor can be achieved withdifferent pairs of spacings and radon release rates. If the sources areto be placed with a relatively high spacing between them, such as 4.5 mmor 5 mm, the sources should have a high radon release rate, such asabove 1.5 microcurie per centimeter length. In contrast, when thespacing between the sources is below 4 mm, the sources may be assigned arelatively low radon release rate.

Given the selected safety factor range, a suitable source spacing isselected. As stated above, the largest spacing which is still believedto destroy the tumor with seeds having an activity level which is nottoo high, is selected. Applicant is limiting the election of spacings tosteps of 0.5 millimeters, which is believed to be close to a level ofinaccuracy in seed placement. These inaccuracies are taken intoconsideration in the safety factor.

After selecting the spacing, a range of radon release ratescorresponding to the spacing and to the safety factor is selected. Thisrange of radon release rates is believed to provide best results intreating tumors of the tumor type for which the calculations wereperformed. It is noted that the selected range of radon release rates isnot limited to use with the specific spacing used to select the range,but rather can be used, due to the safety margin, with a range ofspacings surrounding the selected spacing.

TABLE 1 Effective long-term Required dose diffusion length (Biologicalin millimeters effective dose (BED) Tumor type (all sizes) in Grayequivalent) Squamous cell carcinoma 0.44 60 Colorectal 0.44 120Glioblastoma (GBM) 0.27 100 Melanoma 0.40 150 Prostate 0.32 173 Breast(triple negative) 0.35 60 Pancreatic cancer 0.29 100

As stated in Table 1, the effective long-term diffusion length forpancreatic cancer is estimated to be about 0.29 mm and the required doseis about 100 GyE.

Table 2 presents the beta dose, the corresponding required alpharadiation dose, the estimated alpha radiation dose and the resultingsafety factor, for several spacings and radon release rates forpancreatic cancer. FIG. 7 is a graph which shows the safety factor forvarious spacings and a range of radon release rates for pancreaticcancer, in accordance with embodiments of the present invention.

From FIG. 7 applicant determined that a spacing of 4 millimeters wouldrequire seeds with a radon release rate substantially above 2.5microcurie. To avoid such a high activity level, a spacing of 3.5millimeters is assumed in selecting the radon release rate for thesources. The actual spacing used is optionally shorter than 3.9millimeters, shorter than 3.8 millimeters, shorter than 3.7 millimeters,or even shorter than 3.6 millimeters. On the other hand, the actualspacing used is optionally greater than 3.1 millimeters, greater than3.2 millimeters, greater than 3.3 millimeters, or even greater than 3.4millimeters.

TABLE 2 Pancreas cancer Spacing (mm) 3 3.5 4 Beta dose 0.9 μCi 16.3 11.58.0 1.35 μCi 24.4 17.3 12.0 1.8 μCi 32.6 23.1 16.0 2.25 μCi 40.7 28.820.0 2.7 μCi 48.9 34.6 24.0 Required 0.9 μCi 16.7 17.7 18.4 nominal 1.35μCi 15.1 16.5 17.6 alpha dose 1.8 μCi 13.5 15.4 16.8 2.25 μCi 11.9 14.216.0 2.7 μCi 10.2 13.1 15.2 alpha dose 0.9 μCi 54.8 18.8 6.5 1.35 μCi82.1 28.3 9.8 1.8 μCi 109.5 37.7 13.0 2.25 μCi 136.9 47.1 16.3 2.7 μCi164.3 56.5 19.5 Safety factor 0.9 μCi 3.3 1.1 0.4 1.35 μCi 5.4 1.7 0.61.8 μCi 8.1 2.4 0.8 2.25 μCi 11.6 3.3 1.0 2.7 μCi 16.1 4.3 1.3

For the 3.5 millimeter spacing, a radon release rate of between 1.2 and2.5 microcurie per centimeter length is selected to achieve sufficientdestruction of the tumor, without exposing the patient to unrequiredradiation. For a long-term treatment, this corresponds to a cumulatedactivity of released radon of between about 5.6 MBq hour per centimeterand 11.6 MBq hour per centimeter.

In some embodiments, in order to increase the probability of success ofthe treatment, a radon release rate of at least 1.4 microcurie percentimeter length, at least 1.5 microcurie per centimeter length, atleast 1.7 microcurie per centimeter length or even at least 1.8microcurie per centimeter length is used for pancreatic cancer. On theother hand, in some embodiments, in order to reduce the amount ofradiation to which the patient is exposed, the radon release rate is notgreater than 2.2, not greater than 2.0, not greater than 1.8 or even notgreater than 1.75 microcurie per centimeter length. In otherembodiments, a safety factor of between 1.5-2.5 is used, and accordinglythe radon release rate is between 1.2 and 1.85 microcurie per centimeterlength. In still other embodiments, a safety factor of between 3-4 isused, and accordingly the radon release rate of the seeds 21 is between2.1 and 2.5 microcurie per centimeter length.

Alternatively or additionally, the sources optionally include at least5.9 MBq hour per centimeter, at least 6.4 MBq hour per centimeter, atleast 6.8 MBq hour per centimeter or even at least 7.3 MBq hour percentimeter. On the other hand, the sources optionally include less than10.5 MBq hour per centimeter or even less than 9 MBq hour percentimeter.

In some cases, for example when a spacing lower than 3.5 millimeters,such as about 3.2 millimeters, can be achieved with reasonable accuracy,the radon release rate of the seeds may be on the lower part of theabove range, for example less than 1.5 microcurie per centimeter length,or even less than 1.4 microcurie per centimeter length.

CONCLUSION

It will be appreciated that the above described methods and apparatusare to be interpreted as including apparatus for carrying out themethods and methods of using the apparatus. It should be understood thatfeatures and/or steps described with respect to one embodiment maysometimes be used with other embodiments and that not all embodiments ofthe invention have all of the features and/or steps shown in aparticular figure or described with respect to one of the specificembodiments. Tasks are not necessarily performed in the exact orderdescribed.

It is noted that some of the above described embodiments may includestructure, acts or details of structures and acts that may not beessential to the invention and which are described as examples.Structure and acts described herein are replaceable by equivalents whichperform the same function, even if the structure or acts are different,as known in the art. The embodiments described above are cited by way ofexample, and the present invention is not limited to what has beenparticularly shown and described hereinabove. Rather, the scope of thepresent invention includes both combinations and subcombinations of thevarious features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art. Therefore, the scope of the invention is limited only bythe elements and limitations as used in the claims, wherein the terms“comprise,” “include,” “have” and their conjugates, shall mean, whenused in the claims, “including but not necessarily limited to.”

APPENDIX A Effective Diffusion Length Measurement

A single DaRT seed (6.5 mm length, 0.7 mm outer diameter), carrying 2-3uCi ²²⁴Ra, is inserted to the center of a mice-borne tumor 7-20 daysafter tumor inoculation, when the tumor transverse diameter is ˜6-15 mmFour to five days later, the tumor is excised (as a whole) and cut intwo halves, at the estimated location of the seed center, perpendicularto the seed axis. The seed is then pulled out using surgical tweezersand placed in a water-filled tube for subsequent measurement by a gammacounter. The tumor is kept for one hour at −80° C. It is then taken, indry ice, for measurement in the same gamma counter to determine the²¹²Pb activity it contains. The measurements of the seed and tumoractivity are used to determine the ²¹²Pb leakage probability from thetumor.

Immediately after the gamma measurement, both halves of the tumorundergo histological sectioning by a cryostat microtome. Sections arecut at 250-300 μm intervals with a thickness of μm, laid on positivelycharged glass slides and fixed with 4% paraformaldehyde. Typically thereare 5-15 sections per tumor, spanning a length of 1.5-5 mm Shortly aftertheir preparation, the glass slides are placed, faced down, for aduration of one hour, on a phosphor imaging plate (Fujifilm TR2040S)protected by a 12 μm Mylar foil and enclosed in a light-tight casing.Alpha particles emitted from the sections in the decays of ²¹²Pb progenyatoms, ²¹²Bi and ²¹²Pb, penetrate through the foil and deposit energy inthe active layer of the phosphor imaging plate. The plate is then readout by a phosphor-imaging scanner (Fujifilm FLA-9000).

For each tumor section, the result is a two-dimensional intensity map,proportional to the local ²¹²Pb activity. The intensity (in units ofphoto-stimulated luminescence) is converted to ²¹²Pb activity usingsuitable calibration samples measured concurrently with the slides. Thepoint where the seed crosses the section is identified either by theappearance of a “hole” in the activity map, or by taking thecenter-of-gravity of the activity distribution. Examples are shown inFIGS. 8A-8D. We define a region of interest (ROI) centered at theestimated seed location, and divide it into 0.1 mm-wide concentricrings, with radii in the range 0.5-3 mm. For each ring, we calculate themean value of the activity. If the ROI extends beyond the area of thetumor section, or includes a region with degraded tissue or imagequality, the ring average is taken over a limited azimuthal sector. Theresulting curve of the activity as a function of the radial distancefrom the origin (estimated seed location), is then fitted numerically bya function describing the radial activity distribution from a seed,based on the diffusion-leakage model. The calculation describes the seedas a line source perpendicular to the image. The source is divided intoa large number of point-like segments, each contributing, to a givenpixel in the plane of the image, an activity

$A{\frac{\exp\left( {- r/L_{eff}} \right)}{r}.}$

In this expression r is the distance between the source segment and thepixel under consideration, and A and L_(Pb) are free parameters, whosevalues are adjusted to optimize the fit to the entire curve (FIGS.8A-8D). The value obtained for L_(eff) is taken as an estimate for theeffective diffusion length of the section. The average value of L_(eff)over all sections is taken to represent the effective (or dominant)diffusion length of the tumor, with an uncertainty equal to the standarddeviation of the values obtained in all sections.

FIG. 8A shows a spatial distribution of the photo-stimulatedluminescence (PSL) signal in a histological section of a 4T1 tumor withdenoted region of sampled data in white (0-4 mm) and the fit region withmagenta dashed line (0.5-3 mm). The seed position is determinedmanually.

FIG. 8B is a graph of the radial activity distribution for the sampleddata of FIG. 8A, fitted by the diffusion-leakage model.

FIG. 8C shows a PSL spatial distribution in another histological sectionfrom the same tumor where the seed position is determined automaticallyby calculating the intensity center-of-gravity.

FIG. 8D is a graph of the radial activity distribution for the sampleddata of FIG. 8C, fitted by the diffusion-leakage model.

FIG. 9 shows the measured values of the effective diffusion length as afunction of tumor mass for pancreatic tumors.

FIG. 10 shows the measured values of the effective diffusion length as afunction of tumor mass for prostate tumors.

FIG. 11 shows the measured values of the effective diffusion length as afunction of tumor mass for melanoma tumors.

FIG. 12 shows the measured values of the effective diffusion length as afunction of tumor mass for squamous cell carcinoma tumors.

FIG. 13 shows the measured values of the effective diffusion length as afunction of tumor mass for triple negative breast tumors.

FIG. 14 shows the measured values of the effective diffusion length as afunction of tumor mass for GBM tumors.

APPENDIX B Rn Measurement Methodology

A DaRT seed is inserted to a tumor for a relatively short time—30minutes, after which the seed is removed (in order to prevent the Pbbuildup inside the tumor). The tumor is then set to freeze and cut into10 μm-thick sections perpendicular to the seed axis. These are placed onglass slides and are fixed using Formaldehyde. The tumor sections arethan taken to a digital autoradiography system (iQID alpha camera, byQScint Imaging Solutions, LLC), which records alpha particle hitsone-by-one, providing their xy coordinates (with an accuracy of ˜20 μm),a timestamp and a signal proportional to the deposited energy.

An example for an image, consisting of four histological sections of aDaRT treated tumor and acquired using the iQID system is shown in FIG.15 , which shows four histological sections of a DaRT treated tumor,acquired using the iQID autoradiography system. For the analysis, theimage is cropped so that each section is analyzed independently, as canbe seen in FIG. 16A, which shows a single histological section used forthe analysis. For each section, the center is chosen (either by acenter-of-gravity method, or by identifying a “hole” in the activitymap), and the average number of alpha particle counts is calculated atincreased radial distances from the center. The resulting plot is thenfitted numerically, by assuming that the recorded activity map is asuperposition of infinitesimal segments along the DaRT seed, where eachsegment is calculated using eq. 1:

$\begin{matrix}{{\Gamma(r)} = {\frac{A}{r} \cdot e^{({- \frac{L_{Rn}}{r}})}}} & (1)\end{matrix}$

In this expression, r is the radial distance between the seed segmentand the point-of-interest on the image, L_(Rn) is the radon diffusionlength and A is a free parameter. These two parameters (L_(Rn), A) arefound by a least-squares-fit approach.

The fit is performed over a limited region of the activity distribution,to avoid the artificial “hole” in the center (where the DaRT seed was)and the far end of the distribution, where the statistical variation aretoo large. An example of a fitted curve is shown in FIG. 16B, whichshows the calculated average counts as a function of distance from theseed location, including the fitted function.

1. A method for treating a pancreatic cancer tumor, comprising:identifying a tumor as a pancreatic cancer tumor; and implanting in thetumor identified as a pancreatic cancer tumor, as least one diffusingalpha-emitter radiation therapy (DaRT) source with a suitable radonrelease rate and for a given duration, such that the source providesduring the given duration a cumulated activity of released radon between5.6 Mega becquerel (MBq) hour and 11.6 MBq hour, per centimeter length.2. The method of claim 1, wherein implanting the as least oneradiotherapy source comprises implanting an array of sources, eachsource separated from its neighboring sources in the array by not morethan 4 millimeters.
 3. The method of claim 2, wherein implanting the asleast one radiotherapy source comprises implanting an array of sourcesin a hexagonal arrangement, each source separated from its neighboringsources in the array by not more than 4 millimeters.
 4. The method ofclaim 1, wherein the as least one radiotherapy source has a radonrelease rate of between 1.2 and 2.5 microcurie per centimeter length. 5.The method of claim 4, wherein the as least one radiotherapy source hasa radon release rate of between 1.4 and 1.9 microcurie per centimeterlength.
 6. The method of claim 1, wherein the method comprises selectingthe given duration before implanting the at least one DaRT source in thetumor, and removing the sources from the tumor after the given durationfrom the implanting of the sources passed.
 7. A method of preparing aradiotherapy treatment, comprising: identifying a tumor as a pancreaticcancer tumor; receiving an image of the pancreatic cancer tumor; andproviding a layout of diffusing alpha-emitter radiation therapy (DaRT)sources for the pancreatic cancer tumor, wherein the sources have aradon release rate of between 1.2 and 2.5 microcurie per centimeterlength.
 8. The method of claim 7, wherein providing the layout comprisesproviding a layout in which a spacing between sources in the tumor is 4millimeters or less.
 9. The method as in claim 7, wherein the sourceshave a radon release rate of between 1.4 and 1.9 microcurie percentimeter length.
 10. An apparatus for preparing a radiotherapytreatment, comprising: an input interface for receiving information on atumor; a processor configured to determine that the tumor is apancreatic cancer tumor and to generate a layout of diffusingalpha-emitter radiation therapy (DaRT) sources for the tumor, whereinthe sources in the layout have a radon release rate of between 1.2 and2.5 microcurie per centimeter length and the sources in the layout arearranged in a regular pattern having a distance between adjacent sourcesof not more than 5 millimeters; and an output interface for displayingthe layout to a human operator.
 11. A method of preparing a radiotherapytreatment, comprising: receiving a request for diffusing alpha-emitterradiation therapy (DaRT) sources for a pancreatic cancer tumor;determining a number of radiotherapy sources required for the pancreaticcancer tumor; and providing a kit including the determined number ofradiotherapy sources, wherein the sources have a radon release rate ofbetween 1.2 and 2.5 microcurie per centimeter length.
 12. The method ofclaim 11, wherein determining the number of required radiotherapysources comprises determining a number of sources required such that thearea of the tumor is covered by sources with a spacing between thesources which is not greater than 4 millimeters.
 13. The method of claim11, wherein the sources have a radon release rate of between 1.4 and 1.9microcurie per centimeter length.
 14. A diffusing alpha-emitterradiation therapy (DaRT) source for implantation in a pancreatic cancertumor, wherein the DaRT source has a radon release rate of between 1.2and 2.5 microcurie per centimeter length.
 15. The DaRT source of claim14, wherein the radon release rate is between 1.4 and 1.9 microcurie percentimeter length.
 16. A kit of diffusing alpha-emitter radiationtherapy (DaRT) source for implantation in a pancreatic cancer tumor,comprising: a package; and a plurality of DaRT sources placed in thepackage, the sources having a radon release rate of between 1.2 and 2.5microcurie per centimeter length.
 17. The kit of claim 16, wherein theradon release rate of the sources is between 1.4 and 1.9 microcurie percentimeter length.
 18. A method for treating a tumor, comprising:identifying a tumor as a pancreatic cancer tumor; and implanting in thetumor identified as a pancreatic cancer tumor, an array of diffusingalpha-emitter radiation therapy (DaRT) sources, in a regular arrangementhaving a spacing between each two adjacent sources of between 3.1 and3.9 millimeters.
 19. The method of claim 18, wherein implanting thearray of sources comprises implanting in a hexagonal arrangement, eachsource separated from its neighboring sources in the array by not morethan 3.7 millimeters.
 20. A diffusing alpha-emitter radiation therapy(DaRT) source for use in treatment of a pancreatic cancer tumor of apatient, the source comprising: a support having a length of at least 1millimeter; and radium-224 atoms coupled to the support such that notmore than 20% of the radium-224 atoms leave the support into the tumorin 24 hours, without decay, when the source is implanted in the tumor,but upon decay, at least 5% of daughter radionuclides of the radium-224atoms leave the support upon decay; characterized in that theadministration pattern of the source comprises implanting the source inthe pancreatic cancer tumor throughout the tumor, with a spacing betweenthe sources of between 3-4.5 millimeters, and the radiation therapysource has a radon release rate of between 1.2 and 2.5 microcurie percentimeter length.
 21. The source of claim 20, wherein theadministration pattern of the source comprises implanting the source inthe pancreatic cancer tumor throughout the tumor, with a spacing betweenthe sources of between 3.1-3.9 millimeters.
 22. The source of claim 20,wherein the radiation therapy source has a radon release rate of between1.2 and 1.85 microcurie per centimeter length.
 23. The source of claim20, wherein the radiation therapy source has a radon release rate ofbetween 1.4 and 1.9 microcurie per centimeter length.
 24. The source ofclaim 20, wherein the as least one radiotherapy source has a radonrelease rate of between 2.1 and 2.5 microcurie per centimeter length.25. The source of claim 20, wherein the administration pattern of thesource comprises implanting the source in the pancreatic cancer tumorthroughout the tumor, in a hexagonal arrangement, each source separatedfrom its neighboring sources in the array by not more than 4millimeters.